When receiving a FinishTrajectory call, the Cartographer nodes catches up on
computing loop closures, runs a final optimization, and when in 2D writes out
a PGM map similar to map_saver.
The 'version_' member of a DrawableSubmap is now properly initialized.
The redundant 'submap_id' in the SubmapQuery response is removed.
The missing slot for reset() is added.